Summary of Significant Accounting Policies - Additional Information (Details) $ in Millions	3 Months Ended		9 Months Ended
	Sep. 30, 2017 USD ($)	Sep. 30, 2016 USD ($)	Sep. 30, 2017 USD ($) Segment	Sep. 30, 2016 USD ($)	Dec. 31, 2016 USD ($)
Significant Accounting Policies [Line Items]
Client reimbursements	$ 1.0	$ 1.4	$ 3.0	$ 4.6
Bad debt expense	0.3	0.0	1.1	0.4
Long term receivables related to private equity and real estate capital advisory engagements	28.5		$ 28.5		$ 26.1
Installments period (in years)			3 years
Depreciation and amortization of property and equipment			Depreciation is computed using the straight-line method over the life of the assets. Amortization of leasehold improvements is computed using the straight-line method over the lesser of the life of the asset or the remaining term of the lease.
Number of business segments | Segment			1
Revenue | Maximum
Significant Accounting Policies [Line Items]
Percent of revenues			1.00%
Aircraft
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets (in years)			7 years
Equipment
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets (in years)			5 years
Furniture and Fixtures
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets (in years)			7 years
Leasehold Improvements
Significant Accounting Policies [Line Items]
Estimated useful lives of fixed assets (in years)			10 years
Estimated useful lives of fixed assets, maximum (in years)			Lesser of 10 years or the remaining lease term
Restricted Stock | Minimum
Significant Accounting Policies [Line Items]
Restricted stock units, amortization service period (years)			4 years
Restricted Stock | Maximum
Significant Accounting Policies [Line Items]
Restricted stock units, amortization service period (years)			5 years
Capital advisory engagements
Significant Accounting Policies [Line Items]
Interest income	$ 0.2	$ 0.1	$ 0.5	$ 0.6